1. Field of the Invention
The present invention relates to a surface wave device utilizing a surface wave of an SH type, whose displacement is mainly displacement in the direction perpendicular to the direction of the surface wave propagation, such as a BGS wave (Bleustein-Gulyaev-Shimizu Wave).
2. Description of the Prior Art
Examples of a surface wave of an SH type whose displacement is mainly displacement in the direction perpendicular to the direction of the surface wave propagation include a BGS wave (also referred to as a piezoelectric shear wave) and a Love wave.
The BGS wave is excited when a material such as piezoelectric ceramics is used to construct, for example, a resonator shown in FIG. 2. In FIG. 2, reference numeral 1 denotes a piezoelectric substrate, which is made of the above described piezoelectric material. Reference numerals 2 and 3 respectively denote comb electrodes. The comb electrodes 2 and 3 respectively have a plurality of electrode fingers 2a and 3a which are inserted into each other. An arrow P indicates an axis of polarization.
In a piezoelectric resonator 4 shown in FIG. 2, if an AC electric field is applied from the comb electrodes 2 and 3, a BGS wave is excited whose displacement is only displacement in the direction perpendicular to the direction A of the surface wave propagation, that is, which has only a shear wave component. The above described piezoelectric resonator using the BGS wave is disclosed in, for example, Proceedings of Conference on the Acoustical Society of Japan, May 1976, pp 351 to 352.
In the surface wave resonator 4 utilizing the BGS wave, the BGS wave is completely reflected from free end surfaces 1a and 1b of the piezoelectric substrate 1. Consequently, reflectors must be formed on regions outside the comb electrodes in a surface wave resonator utilizing a Rayleigh wave, while such reflectors can be omitted in the surface wave resonator 4 utilizing the BGS wave. Accordingly, the surface wave resonator 4 utilizing the BGS wave has the great advantages that its chip size can be significantly decreased to one tenth as compared to the size of the resonator utilizing the Rayleigh wave and it can be applied in the band of high frequencies of 5 MHz to 70 MHz if the precision of the free end surfaces 1a and 1b is increased.
The surface wave resonator 4 utilizing the BGS wave, however, actually has the disadvantage in that ripples and an unnecessary spurious mode occur on impedance-frequency characteristics as shown in FIG. 3 even if the precision of the free end surfaces 1a and 1b is increased. More specifically, as indicated by an arrow B in FIG. 3, ripples and an unnecessary spurious mode occur to a considerable extent in the vicinity of the antiresonance frequency or a lower frequency region. Therefore, a surface wave device utilizing a surface wave of an SH type has not come in practice yet irrespective of the above described great advantages that its chip size can be decreased and it is applicable in a high frequency region.